(1) Field of the Invention
The present invention relates to light-collecting devices, in particular, to a light-collecting device in which a first annular region and a second annular region each having a refractive-index different from each other are adjacently and alternately arranged in a concentric manner, to a light-collecting device group, and to a solid-state imaging apparatus including the light-collecting device group.
(2) Description of the Related Art
An apparatus which converts an image into an electrical signal (the apparatus is called an imaging apparatus) is typically used in equipment electromagnetically recording an image, including digital video recorders, digital still cameras, and camera-equipped cell-phones which are rapidly increasing in recent years. A charge-coupled device sensor (typically, it is called a “CCD sensor”. Hereinafter, it is written as a “CCD sensor”) and a MOS sensor, which are a type of a semiconductor device, have been used for the imaging apparatus in recent years, contributing to miniaturization and price reduction of the imaging apparatus. In the imaging apparatus, plural minute pixels each having therein a single photodetecting device are arranged on a plane so as to form a single screen. Accordingly, the capability of the imaging apparatus is determined by the capability of these plural pixels.
Among the most important capabilities of the imaging apparatus, is the capability to convert a minute input image into an electrical signal with a low noise (i.e., a low S/N ratio), and the capability to output the input image with a high electrical signal (i.e., a high amplification factor).
As a method for achieving the low S/N ratio and high amplification factor, a method for improving the S/N ratio and amplification factor of the photodetecting device in a pixel is firstly considered. In addition, a below-described method is generally adopted.
FIG. 19 is diagram showing a cross section of a single pixel in a typical solid-state imaging apparatus according to a conventional technique. As shown in FIG. 19, a pixel 1601 includes a photodetecting device 1602, a light-collecting device 1603, a color filter 1604, and a line 1606. Incident light 1605 entering the pixel 1601 is collected by the light-collecting device 1603 and separated by a color, such as red, blue, or green by the color filter 1604, followed by being input into the photodetecting device 1602. The density of light-intensity of the incident light 1605 entering the photodetecting device 1602 is increased by the light-collecting device 1603, thereby enabling the low S/N ratio and improvement of the amplification factor.
Here, an incident angle of the incident light 1605 varies, causing a focal point by the light-collecting device 1603 to vary. This results in the incident light to fail to be collected on the photodetecting device 1602. When the pixel 1601 serves as a peripheral pixel in the imaging apparatus, the above problem remarkably occurs.
In order to solve the above problem, there is a conventional technique in which light-collecting devices are asymmetrically arranged with respect to each of pixels (see the patent reference 2: Japanese Unexamined Patent Application Publication No. 2001-196568). Alternatively, in the peripheral pixels in the imaging apparatus, a position of the photodetecting device 1602 has been conventionally displaced with respect to the light-collecting device. However, in these conventional techniques, there has been a problem that a high effect can be obtained with a relatively small incident angle of the incident light 1605, whereas the effect decreases as the incident angle increases. In addition, a position of the line 1606 also needs to be displaced with respect to the photodetecting device 1602 in accordance with the incident angle of the incident light 1605. However, the position displacement may normally be impossible due to a restriction of a circuit layout (layout rule).
In order to achieve a solution that characteristics of the pixel is maintained even when the incident angle is large, the patent reference 1 (International Patent Publication No. WO 2005/101067) discloses the light-collecting device formed as shown in FIG. 20A. The pixel shown in FIG. 20A includes a plurality of annular-shaped light-transmitting films 1501, a substrate 1502, a photodetecting device 1504, and a line 1506. The light-transmitting films 1501 are formed in a circular shape or the annular shape in a concentric manner. A width of the annular region is at a level of a wavelength of a natural light or narrower than that. A typical width is at a level of 0.1 μm. The refractive index reacting on the incident light 1505 passing through the light-transmitting film 1501 falls in an averaged value in the range of the wavelength on a surface of the light-transmitting film 1501, but does not fall in the value of the refractive index of the light-transmitting film 1501 or of a medium (typically, air). Since the annular region is extremely narrow in width, the refractive index reacting on the incident light 1505 depends on the width of the annular region. Accordingly, the refractive index falls in an intermediate value between the refractive index of the light-transmitting film 1501 and that of the medium. Specifically, for the incident light 1505, the refractive index is concentrically spread on a surface of the light-transmitting film 1501. Appropriate arrangement of the spread of the refractive index allows the incident light 1505 having passed through the light-transmitting film 1501 and the substrate 1502 to be collected by the diffraction effect so as to reach the photodetecting device 1504. A position where the incident light 1505 is collected can be controlled by varying a shape of the light-transmitting film 1501. Accordingly, the incident light can be collected in the photodetecting device 1504 without deteriorating the capability thereof by designing the shape of the light-transmitting film 1501 taking the incident angle of the incident light 1505 into account. Therefore, the above-mentioned solution can be achieved.
FIG. 20B is a top view of FIG. 20A. The reference numerals 1501 and 1507 respectively denote the light-transmitting film and the medium each having an annular shape and being concentrically arranged.